Abstract
The mathematical model for evolution of legume-rhizobia mutualism based on the partners' positive feedbacks resulted from their metabolic (C-N) exchange is presented. Negative FDS in rhizobia population, combined with the partners' positive feedbacks ensure anchoring or even domination of the mutants which either acquired the mutualistic traits or changed the specificity in their expression with different host genotypes. The created model allows us to consider the mutualistic symbiosis as of a finely balanced population system in which the equilibrium may be shifted in favor of beneficial microbial genotypes due to natural selection for an improved symbiotic efficiency implemented in plant population. Research is supported by RFBR grant 06-04-48800a.
Highlights
Beneficial symbioses represent a highly efficient strategy for adaptation of organisms to the changing and stressful environments (Douglas, 1994)
N2 fixed from atmosphere by root nodule bacteria or phosphates assimilated from soil by arbuscular mycorrhizal fungi are donated from micro-symbionts to hosts and specialization for implementing these functions may be very deep, leading to a restriction of reproductive ability as it is typical for glomalean fungi (Smith and Read, 1997) and for rhizobial bacteroids (Brewin, 2004)
The facultative mutualism represents an evolutionary enigmatic phenomenon since in many symbiotic systems, partners maintain the genes which benefit the co-habitants even at the expense of survival in the owners of these genes/traits. It was argued repeatedly (Frank, 1996; Douglas, 1998; Simms and Taylor, 2002; Gundel et al, 2007) that mutualistic traits should be stable in the microbes which are involved in the obligatory symbioses and are vertically transmitted in host generations (“forced altruism” based on the full dependency of both partners on mutualistic interactions; Darlington, 1978)
Summary
Beneficial (mutualistic) symbioses represent a highly efficient strategy for adaptation of organisms to the changing and stressful environments (Douglas, 1994). The positive partners’ feedbacks are described assuming that: (i) plants and bacteria are represented by dimorphic populations the components of which combine randomly into symbiotic systems differing in ability to fix N2; (ii) fitness (reproductive success) in both partners is determined by the efficiency of N2-fixation. This approach enables us to study the interplay of FDS and Darwinian selection in co-evolving partners’ populations which results in anchoring and even domination of “genuine mutualists” (N2-fixing strains) over the “symbiotic cheaters” (non-fixing strains) maintained at an equilibrium in “plant-soil” system
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